This invention relates to the production of ammonium polyphosphate. It has particular, but not exclusive, application to producing the type of water-insoluble ammonium polyphosphate useful in fire- and heat-resisting materials.
The term ammonium polyphosphate is used to describe a family of compounds, ranging from water soluble, low molecular weight materials utilized as fertilizers, to highly water insoluble long chain molecules used as starting materials in various manufacturing processes, as fire-fighting materials, and as additives to coatings to give the coatings fire-resistant and heat-resistant properties.
Numerous methods have been proposed and used for producing ammonium polyphosphate. Liquid fertilizer compositions have been formed from direct ammoniation of superphosphoric acid (a mixture of orthophosphoric acid and polyphosphoric acid) with anhydrous ammonia. For example, in Hignett et al, U.S. Pat. No. 3,171,733, superphosphoric acid (76% P.sub.2 O.sub.5) and anhydrous ammonia are reacted at elevated temperatures and controlled pressure. The process requires continuous agitation and is completed in a time period of one hour or more. The product is a water soluble, linear, low molecular weight material.
In many applications, it is highly desirable to utilize an ammonium polyphosphate having a water solubility of less than about two percent. Materials have been known for about twenty years, having a solubility of about 1.5 percent and are known as "substantially water-insoluble" ammonium polyphosphates. They are believed to be crosslinked, high molecular weight, crystalline materials. Commercial substantially water-insoluble ammonium polyphosphate is sold by Monsanto Company under the designation "PhosChek P--30." It may be noted that the commercial material gives two distinct and almost equal thermochemical decompositions, and is believed to be formed by reaction of urea with orthophosphoric acid. It is therefore believed that the material may actually be a chain incorporating both ammonium polyphosphate and either cyanuric acid or cyamelide in nearly equal proportions, although this surmise has not yet been conclusively demonstrated. The term "substantially water-insoluble ammonium polyphosphate" as used herein includes this material. The substantially water-insoluble ammonium polyphosphates are high molecular weight materials, characterized by P--O--P bonding.
The known processes for forming crystalline, substantially water-insoluble ammonium polyphosphates are generally characterized by forming the polyphosphate backbone in situ. For example, Shen et al, U.S. Pat. No. 3,397,035, produces the long-chain material by mixing about a 1:1 ratio of substantially water-insoluble crystalline ammonium polyphosphate with a reaction mixture of inorganic phosphate and an inorganic source of ammonia, and heating the mixture. These processes are slow, difficult to control, give limited yield, and require the expenditure of large amounts of energy.